Wan grey light filters through the panes of Paddington station as Molly waits for Piotr to arrive

It's raining in London. The sky over Paddington Station looks the colour of cigarette smoke, Molly thinks; it's noon, but a wan grey light filters through the translucent panels in the ceiling. Water is trickling through the leaky Victorian superstructure above her, dripping into barrels and buckets covered in blue netting.

She's sitting at a cafe table in the station's food court, not far from the statue of Paddington Bear, waiting for help to arrive. Molly's nervous. She's glad she picked a public place for the meeting; it's a busy weekday, with plenty of families starting half-term breaks. If it doesn't go well, she'll be able to slip off into the crowd.

Molly isn't sure who to trust. The police aren't an option, not since her lunch date with rock-star detective Martin Drake. She loves her Dad dearly but in some ways he's more helpless than she is. The few other relatives she has are scattered over three different continents. In the end, it was ACENET that she went to; it still feels like a leap of faith.

When she'd explained to the site administrator the kind of help she was going to need, Legba had emailed her back almost immediately:

Molly: I was so sorry to hear about Danny. At the risk of giving away too much about my own background - I do have to be careful to preserve my air of mystery, after all - Danny helped me set up ACENET. Even as a teenager, he was one of the cleverest network guys that I'd ever come across; one of the best and the brightest. I know that he saw the same thing in you.

I don't want to know too much about what you're proposing to do – it's safer for everyone that way – but I'll help in whatever way I can. Danny had other friends like you, and I'll put you in touch with them. I think you should certainly speak to Piotr. His contact details are coming through shortly.Keep in touchL

She's wondering what Piotr Sevchenko is going to be like. Molly would rather solve all her problems herself, but having thought long and hard about the fingerprint scanner, she can't see a way round it; Piotr's ACENET profile calls him a 'Security Consultant'. She's not exactly sure what that means, but it sounds like a good fit. Whether he'll be willing to help her break the law is another matter.

It's ten minutes past 12. He's ten minutes late. Where is he? Someone taps her shoulder from behind, and Molly almost chokes on her orange juice. She turns her head and sees a tall youth in a leather jacket, looming over her, grinning.

"It's not such a good table, that you have chosen! Off to the side is better. Easier to see all directions of approach. You are Molly Root? I am pleased to meet you!" He has a cheerful tenor voice, and a faint but distinct Russian accent.

He puts out his hand, and Molly shakes it. "Yes! Piotr, right?"

"Piotr Sevchenko. Sorry to startle you. Next time, I approach from front, with a carnation in my buttonhole." He lets go of Molly's hand, turns his own right hand over, turns it back, and there's a thin grey rectangle in it where there wasn't one before. "My card."

Molly picks it up. It's not card at all - some sort of springy, lightweight plastic. On one side is printed Piotr's name, in Roman and Cyrillic letters, above the words "SECURITY SERVICES".

"Also magic," says Piotr.

***

Piotr sips at a coffee while Molly explains about Danny. She likes him already. It's a relief, after two days of worrying at this on her own, to be able to talk to someone; and Piotr looks like he believes her.

He's 19, according to his profile, but looks a little younger, with too-long legs that he seems still to be getting used to. He's over six feet, towering over Molly, and topped with a shock of brown wavy hair that constantly has to be brushed back from his eyes. The eyes are blue, and sharp; a watchmaker's.

She tells him about the fire, about Danny's disappearance, and about finding his phone and files. Molly hasn't quite decided how much to tell Piotr about Damocles, but before she has to make up her mind, Piotr interrupts her.

"These are files that he has stolen, I am guessing, part of his big crusade against evil? Damocles stuff."

"Yes – you know about that?"

"Sure. I didn't encourage it, but I know a lot of people who work on both sides of the fence. The black hats, the white hats, the grey hats, you know? Danny was political, he cared about things a lot, maybe too much. I told him not to involve me, and when he was working for me, to do it right – and he did. I didn't have any complaints."

Molly looks at Piotr's card, on the table in front of her. "So he did, what - security services for you?"

Piotr smiles. "Do you know what a Red Team is?" Molly shakes her head. "Red Team is for companies to test security. How good their systems are. They don't want spies or crackers stealing their secrets. So you set a thief to catch a thief.

"Company calls me. I try to break in to their building, get access to their computers, steal the lunch menu from the safe - whatever they are scared about. They pay me to do this. So I put together my Red Team. Sometimes Danny would do a job for me."

"What sort of things?" Molly asks.

"Well, one thing works a lot. You know the little flash drives - ah, like the one Danny left for you. Yes. You take a few of those, scatter them around the company parking lot, or in the café where the office workers have lunch. Maybe I leave one here where the Chairman gets his coffee on the way to work. So they find this little stick, and they want to know what is on it. It is only natural curiosity, right?"

"Right," says Molly.

"And what is on it but one of Danny's clever little Trojan horses. They plug it in to the office machine, it takes over the network so easily, and that is often the end of the operation, because we know all their secrets." Piotr stirs brown sugar into his third coffee. "And I collect my fee."

"Piotr, that's exactly what I need. The files Danny stole? There's a police detective looking for them, and I don't think he's working a case. I think he's working for whoever's behind this."

Molly reaches into her bag and gets her slate. The pictures and video she has taken of Harris, Renfield and Church are all on there. She puts it between them and starts swiping through them. "He passed them on to someone who I think works here. I've been inside, but I didn't have the chance to find out who, or why.

"I need to go back in, and look at their databases - find out who is paying for this. There has to be some kind of trail."

Piotr's eyes narrow. "You have tried cracking wireless?"

"I've cracked it, but the systems I want are terminal access only, I think. It looks like just email over wi-fi."

"Ah," Piotr says. "Then we must go in. But I am thinking that this is highly illegal, and you are very young - "

"Hey!" snaps Molly.

"-and so am I, yes, I know." They sit there for a moment, looking at each other.

"If we don't do something about Danny," Molly says, eventually, "then who will?"

Piotr brushes the hair out of his eyes. "Da," he says.

The next instalment of Root will be available on FridayIf you can't wait till then, join the discussion on our Facebook page

Her best friend has been murdered, revealing his secret life as a notorious hacker, and suspicion falls on a corporation with shady secrets to hide. Teenage computer expert Molly must recruit a highly-skilled team to help her solve the mystery – before it’s too late…

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From 24 October we will publish a new chapter of Root every day for six weeks. But along the way you can influence the story by solving mysteries, foiling plans and eavesdropping into conversations. Find out more at guardian.co.uk/teen-books and join other readers on our Teen Books Facebook page to discuss the story so far and what you think should happen next.